The mass and radius of the neutron star (NS) in low-mass X-ray binaries can be obtained by fitting the X-ray spectrum of the NS in quiescence, and the mass and radius constrains the properties of dense matter in NS cores. A critical ingredient for spectral fits is the composition of the NS atmosphere: hydrogen atmospheres are assumed in most prior work, but helium atmospheres are possible if the donor star is a helium white dwarf. Here we perform spectral fits to XMM, Chandra, and ROSAT data of a quiescent NS in the globular cluster M13. This NS has the smallest inferred radius from previous spectral fitting. Assuming an atmosphere composed of hydrogen, we find a significantly larger radius, more consistent with those from other quiescent NSs. With a helium atmosphere (an equally acceptable fit), we find even larger values for the radius.
@article{arxiv.1301.3768,
title = {Mass/Radius Constraints on the Quiescent Neutron Star in M13 Using Hydrogen and Helium Atmospheres},
author = {Andrei Catuneanu and Craig O. Heinke and Gregory R. Sivakoff and Wynn C. G. Ho and Mathieu Servillat},
journal= {arXiv preprint arXiv:1301.3768},
year = {2015}
}